U.S. patent number 6,761,777 [Application Number 10/040,357] was granted by the patent office on 2004-07-13 for high chromium nitrogen bearing castable alloy.
Invention is credited to Roman Radon.
United States Patent |
6,761,777 |
Radon |
July 13, 2004 |
High chromium nitrogen bearing castable alloy
Abstract
The present invention is directed to a corrosion and erosion
resistant High Chromium, Nitrogen bearing alloy, comprising the
following composition in wt. %: 28-48 chromium, 0.01-0.7 nitrogen,
0.5-30 manganese, 0.01-5 boron, 0.3-2.5 carbon, up to 0.01-25
cobalt plus nickel, up to 0.01-5 silicon, up to 0.01-8 copper, up
to 0.01-6 molybdenum, up to 2% of each one selected from group
consisting of zirconium, vanadium, cerium, titanium, tungsten,
niobium, aluminum, calcium, and rare earth elements with the
balance being essentially iron and other trace elements or
inevitable impurities. The alloy has a microstructure comprising
hypoeutectic, eutectic, chromium carbides, boride and nitrides in
the austenitic matrix, saturated with nitrogen with virtually no
secondary carbides and nitrides.
Inventors: |
Radon; Roman (Belleview,
FL) |
Family
ID: |
21910562 |
Appl.
No.: |
10/040,357 |
Filed: |
January 9, 2002 |
Current U.S.
Class: |
148/325; 148/324;
148/327; 420/12; 420/11; 420/121; 420/65; 420/64; 420/59; 420/56;
420/128 |
Current CPC
Class: |
C22C
38/52 (20130101); C22C 38/44 (20130101); C22C
38/001 (20130101); C22C 38/54 (20130101); C22C
38/58 (20130101); C22C 30/00 (20130101); C22C
38/42 (20130101) |
Current International
Class: |
C22C
38/52 (20060101); C22C 38/42 (20060101); C22C
38/58 (20060101); C22C 38/00 (20060101); C22C
38/18 (20060101); C22C 30/00 (20060101); C22C
38/44 (20060101); C22C 37/06 (20060101); C22C
38/54 (20060101); C22C 37/00 (20060101); C22C
038/18 () |
Field of
Search: |
;148/324,325,327
;420/11,12,56,59,64,65,121,128 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Oltmans; Andrew L.
Attorney, Agent or Firm: Greenblum & Bernstein,
P.L.C.
Claims
What I claim is:
1. A corrosion and erosion resistant castable alloy comprising, in
% by weight: 31 to 48 chromium 0.01 to 0.7 nitrogen 0.5 to 30
manganese 0.3 to 2.5 carbon 0.01 to 5 boron 0 to 6 molybdenum 0 to
5 silicon 0 to 8 copper 0 to 4 cobalt 0 to 25 nickel plus
cobalt,
said alloy further comprising 0 to 2% of each of one or more of
zirconium, vanadium, cerium, titanium, tantalum, tungsten, niobium,
aluminum, calcium and rare earth elements, the balance comprising
iron and inevitable impurities, said alloy having a microstructure
comprising chromium carbides, borides and nitrides in an austenitic
matrix, said matrix having a face centered cubic crystal structure
and being supersaturated with nitrogen in solid solution form, the
composition of the alloy satisfying the equation: ##EQU3##
2. The alloy of claim 1, wherein the alloy comprises of at least
one of molybdenum, silicon, copper and (nickel plus cobalt), each
in an amount of at least 0.01% by weight.
3. The alloy of claim 1, wherein the alloy comprises at least 32%
by weight of chromium.
4. The alloy of claim 3, wherein the alloy comprises of at least
one of molybdenum, silicon, copper and (nickel plus cobalt), each
in an amount of at least 0.01% by weight.
5. The alloy of claim 3, wherein the alloy comprises, in % by
weight: 32 to 34 chromium 0.35 to 0.45 nitrogen 6 to 9 manganese
0.5 to 2.5 carbon 0.01 to 4.5 boron 0 to 5 molybdenum 0 to 3
silicon 0 to 4 copper 0 to 4 nickel plus cobalt,
the balance comprising iron and inevitable impurities.
6. The alloy of claim 5, wherein the alloy comprises, in % by
weight, one or more of the following: 2 to 5 molybdenum 0.5 to 3
silicon 1 to 4 copper 2 to 4 nickel plus cobalt.
7. The alloy of claim 5, wherein the matrix comprises 0.35% by
weight of nitrogen in solid solution form.
8. The alloy of claim 6, wherein the matrix comprises 0.35% by
weight of nitrogen in solid solution form.
9. The alloy of claim 3, wherein the alloy comprises, in % by
weight: 35 to 40 chromium 0.4 to 0.6 nitrogen 6 to 15 manganese 0.8
to 1.5 carbon 0.01 to 4 boron 0 to 5 molybdenum 0 to 3 silicon 0 to
6 copper 0 to 12 nickel plus cobalt,
the balance comprising iron and inevitable impurities.
10. The alloy of claim 8, wherein the alloy comprises, in % by
weight, one or more of the following: 2 to 5 molybdenum 0.5 to 3
silicon 1 to 6 copper 4 to 12 nickel plus cobalt.
11. The alloy of claim 9, wherein a PREN is from 58 to 66.
12. The alloy of claim 10, wherein a PREN is from 58 to 66.
13. The alloy of claim 11, wherein the matrix comprises 0.4% by
weight of nitrogen in solid solution form.
14. The alloy of claim 12, wherein the matrix comprises 0.4% by
weight of nitrogen in solid solution form.
15. The alloy of claim 3, wherein the alloy comprises, in % by
weight: 41 to 48 chromium 0.45 to 0.7 nitrogen 6 to 30 manganese
0.9 to 1.5 carbon 0.01 to 3.5 boron 0 to 4 molybdenum 0 to 3
silicon 0 to 8 copper 0 to 25 nickel plus cobalt,
the balance comprising iron and inevitable impurities.
16. The alloy of claim 15, wherein the alloy comprises, in % by
weight, one or more of the following: 1 to 4 molybdenum 0.5 to 3
silicon 1 to 8 copper 10 to 25 nickel plus cobalt.
17. The alloy of claim 15, wherein a PREN is from 51 to 72.
18. The alloy of claim 16, wherein a PREN is from 51 to 72.
19. The alloy of claim 17, wherein the matrix comprises 0.45% by
weight of nitrogen in solid solution form.
20. The alloy of claim 18, wherein the matrix comprises 0.45% by
weight of nitrogen in solid solution form.
21. A casting which comprises the alloy of claim 1.
22. A part of a slurry pump which comprises the alloy of claim
1.
23. The part of claim 22, wherein the part comprises one of a
casing, impeller, suction liner, pipe, nozzle, agitator and a valve
blade.
24. The casting of claim 21, wherein the casting comprises the
alloy of claim 10.
25. A casting which comprises the alloy of claim 10.
Description
This application was originally deposited on Aug. 6, 2001, in the
United States Patent and Trademark Office under the Disclosure
Document Deposit Program and was assigned Disclosure Document No.
497,934.
FIELD OF INVENTION
This invention relates generally to the art of alloys and more
particularly to a high chromium, nitrogen bearing alloy having high
corrosion resistance. The instant invention also relates to a high
chromium-nitrogen bearing castable alloy, a high chromium-nitrogen
content alloy, and a process for producing the high
chromium-nitrogen bearing alloy, and articles prepared from the
same. This invention further relates to a corrosion resistant high
chromium, nitrogen bearing austenitic alloy which is also excellent
in strength at high temperatures and suitable for materials of
boilers, chemical plant reactors and other apparatus which are
exposed to severely high temperature and corrosion environments at
work. The instant invention is also directed to a heat resistant
high Chromium, nitrogen bearing austenitic alloy having high
strength and excellent corrosion resistance in high temperature
corrosive environments. The present also addresses the problem of
creating a metal casting material, the wear resistance of which
will correspond approximately to common commercial types of white
iron, but which additionally will be characterized by high
corrosion resistance in aggressive media. In addition to high
corrosion and wear resistance, the alloy material according to the
invention has good casting characteristics. Consequently it can be
produced in conventional high-grade steel foundries. Moreover, the
casting material has good working characteristics. Furthermore, the
aforementioned positive quantities are primarily a chromium content
of 28 to 48 wt. %, a carbon content of 0.3 to 2.5 wt. %, and a
nitrogen content of 0.01 to 0.7% which result in a sufficiently
high volume proportion of carbides and nitrides. The large increase
of the chromium content decreases the chromium depletion of the
matrix. With regard to the combination of corrosion resistance and
wear resistance, the material according to the invention is
decidedly superior compared to the known types of castings
previously utilized in applications subjected to hydroabrasive
wear. The present invention is also directed to an air-meltable,
castable, workable, alloy resistant to corrosion and acids such as
sulfuric acid and phosphoric acid over a wide range of acid
strengths.
BACKGROUND OF INVENTION
Equipment used in highly corrosive environments typically is
constructed of metal alloys such as stainless steel or other high
alloys. These alloys are necessary to withstand the extremely
corrosive effects of environments in which the equipment encounters
chemicals such as concentrated sulfuric acid or concentrated
phosphoric acid. A particularly difficult environment is
encountered in making phosphate fertilizer. In the digestion of
phosphate rock with hot, concentrated sulfuric acid, equipment must
resist the environment at temperatures up to about 100.degree. C.
The impure phosphoric acid which is produced can be extremely
corrosive and contains some residual sulfuric acid. The corrosive
effect is often increased by other impurities in the phosphoric
acid, particularly by halogen ions such as chloride and fluoride,
which are normally present in the phosphate rock feedstock used in
the process. An extremely corrosive environment is encountered in
the concentration of the crude phosphoric acid.
Phosphate rock deposits at various locations in the world vary
greatly in chemical composition. The most severe corrosion
environments are typically encountered in processing deposits of
phosphate rock which contain a high content of halogens, such as
chloride or fluoride.
It is also generally known that increasing the Cr content is
effective to improve corrosion resistance of steel. Hi-Chrome
alloys containing 23-40% Cr, 0.8-2% C, 2.5% Si, and up to 5% Mo,
have been known since the 1930's. See for Example German Patent No
7,001,807. U.S. Pat. No. 5,252,149 represents a modernization of
this alloy, followed by the German Patent No. 8,612,044 or No.
4,417,261. It is noted that in both patents the alloys exhibit a
high resistance to abrasion and good resistance to corrosion.
However, both exhibit poor mechanical properties, especially low
toughness, brittleness, sensitivity to heat, sensitivity to notch
all of which limit their usefulness. It is evident that their
structure contains ferrite (Fe .alpha.).
The ferritic structure in these alloys is inherently very brittle,
and the carbide phase embedded in such a brittle phase, results in
a very low toughness, high notch sensitivity, as well as
sensitivity to heat. Besides, the ferritic structure supersaturated
with Chrome, causes the creation of the sigma phase, which
drastically lowers toughness and corrosion resistance.
U.S. Pat. No.5,320,801 is directed to alloys having the following
composition: Cr--27 to 34% by weight, Ni+Co--13 to 31%, Si--3.2 to
4.5%, Cu--2.5 to 4%, C--0.7 to 1.6%, Mn--0.5 to 1.5%, Mo--1 to 4%,
and Fe--essentially the balance. The alloy of the '801 patent
possesses good toughness, but has very poor hardness and very poor
wire resistance and low tensile strength. The hardness of 208 to
354 HB, is similar to that of CD4MCU stainless steel (260-350 HB),
which has excellent corrosion resistance, but poor wear resistance.
The alloy disclosed and claimed in U.S. Pat. No. 5,320,801 is
similar to austenitic, high Nickel stainless steels in that is has
good toughness, but very low tensile strength and hardness, as well
as poor wear resistance. The Nickel present in corrosion resistant
alloys, serves mainly for structural stabilization and adds very
little to their corrosion resistance. Good examples of this are the
stainless austenitic steels containing 12-35% Ni, which have
corrosion resistance approaching that of duplex stainless steels
which have a low percentage of Nickel (4-8%), or High-Chrome
stainless steels with Ni only up to 4%. The primary elements of
stainless alloys are Chromium, Molybdenum and Nitrogen as
illustrated in the models used to show how various alloying
elements influence the corrosion resistance of stainless steel. For
example: Pitting Resistance Equivalent Number, PREN=%
Cr+3.3*Mo+16*% N illustrates that Nitrogen is an important, very
powerful alloying element of corrosion resistant alloys.
The main flaw of the High-Chrome alloys of the prior art is the
difficulty in dissolving of Chrome, Molybdenum and Nitrogen in the
matrix, without a negative effect on the mechanical properties of
the alloy, such as toughness, tensile strength, brittleness, heat
sensitivity and weld ability. This is the result of the
precipitation of the sigma phase from alloys saturated with Chrome
and Molybdenum. Premature wearing out of pump parts made from the
above-mentioned High-Chrome alloys is a common occurrence. The main
contributing factors are: very low toughness, brittleness and low
endurance. Most often a failure happens with a casting worn thin in
an isolated area where, due to the poor mechanical properties of
the alloy, a crack develops leading to the eventual disintegration
of the otherwise still viable component.
The mechanism for corrosion and erosion in acidic environments of
the alloys of the prior art are accelerated corrosion due to the
continuous removal of the passive corrosion resistant layer by
particles in solids containing corrosive fluid. This is especially
evident in alloys containing a higher volume of Chrome and
Molybdenum, where significant amount of sigma phase is unavoidable
and the metal matrix possesses very poor toughness. In order to
restore the passive layer, it is necessary to have the Chrome and
the Molybdenum concentration at as high a level as possible.
Increasing the Chrome/Carbon, or Cr+Mo/C ratio, increases corrosion
resistance up to the critical point, after which begins the
formation of the sigma phase, which drastically reduces the
toughness and lowers the corrosion resistance of the alloy by
depleting the Chrome in the vicinity of the sigma phase
precipitates.
The present invention is based on increasing the ratio expressed by
Cr+N/C-N, or Cr+Mo+N/C and Cr+Mo+N+B/C-N by reducing the Carbon in
the matrix, while introducing the Nitrogen as a powerful additional
alloy element to the High-Chrome alloys where it is in a high
concentration in solid solution.
Nitrogen, like Carbon, forms interstitial solids with
body-centered-cubic (bcc)-.alpha. Iron, and face-centered-cubic
(fcc) .gamma.-iron. The size of the Nitrogen atom is smaller than
that of the Carbon atom; in this case, in the .alpha., as well as
in the .gamma. phases, the Nitrogen occupies the interstitial sites
easier.
The maximum solubility of Nitrogen in Fe-.delta. and Fe-.gamma. is
several times, to tens of times higher than that of Carbon at the
same temperatures, which leads to significant expansion and
distortion of elementary lattices. It has a solid solution
hardening and strengthening effect much greater than that of
Carbon, while maintaining a greater level of toughness.
The solubility limits of Nitrogen in the prior art High-Chrome
alloys are a very low 0.15% N maximum. This limit is dictated by an
inherently low physico-chemical solubility of Nitrogen and Carbon
(0.02 to 0.08 max. C+N) in the structure Fe-.alpha., which
constitutes up to a maximum of 40% of the alloy in German Patent
Nos. 4,417,261 or 8,612,044, as well as the low Manganese content
.ltoreq.1.5%.
The addition of Nitrogen is the most effective means of improving
the mechanical properties of austenitic High-Chrome alloys without
having a deleterious effect on ductility and corrosion resistance.
In order for Nitrogen to be fully effective as an anti-corrosive
agent, and to bring to bear its wide range of positive effects on
the castings' mechanical properties, such as increased tensile
strength hardness and toughness, without loss of ductility,
Applicant discovered that in High-Chrome alloys this can happen
with considerable presence of Manganese and Molybdenum as enhancing
alloys. In these conditions, Nitrogen dissolves in the solid state,
two to four times better than in any other High-Chrome alloy
disclosed in the prior art. Similarly in high Manganese stainless
steels, which dissolve up to 0.8% Nitrogen, and even 1% under
partial to pressure, the tensile strength and the hardness are two
to four times higher, with good ductility than in the same steel
without nitrogen.
The prior art is silent regarding the high-chromium alloys of the
instant invention.
OBJECTS OF THE INVENTION
It is an object of applicants' invention to produce a material of
construction suitable for use in processing such phosphate rock
which presents a severely corrosive environment.
It is also an object of applicants' invention to produce a
corrosion resistant alloy which is high in chromium content and
which has an enhanced corrosion resistance.
It is a further object of applicants' invention to produce a highly
corrosion resistant alloy which contains silicon in sufficient
quantity to render the alloy castable by conventional methods.
It is another object of applicants' invention to produce a highly
corrosion resistant alloy which contains silicon.
Still a further object of applicants' invention is to produce a
corrosion resistant alloy that is high in chromium content and also
contains nitrogen.
It is an additional object of applicants' invention to produce a
corrosion resistant alloy which has high strength and hardness
properties.
An additional object of the present invention is to provide a
High-Chromium, Nitrogen bearing alloy with significant improvement
in mechanical properties.
Yet, another object of the invention is to provide a high-chromium,
nitrogen bearing alloy having greater resistance to corrosion
combined with erosion, particularly in acidic environments
containing chlorides, fluorides media, or other impurities.
A further object of the present invention is to provide a
High-Chromium, Nitrogen bearing alloy containing a large amount of
Nitrogen
It is a further object of the present invention to provide novel
method of hardening a High-Chromium, Nitrogen bearing alloy by
cryogenic treatment.
It is an additional object of applicants' invention to produce a
High Chromium, Nitrogen and Boron containing alloy which is erosion
and corrosion resistant.
SUMMARY OF THE INVENTION
The instant invention is also directed to a corrosion and erosion
resistant high-chromium nitrogen bearing and castable alloy
comprising the following composition in wt. %: 28% to 48% Chromium
0.01% to 0.7% Nitrogen 0.5% to 30% Manganese 0.3% to 2.5% Carbon
0.01% to 5% Boron optionally 0.01% to 6% Molybdenum optionally
0.01% to 5% Silicon optionally 0.01% to 8% Copper optionally 0.01%
to 25% Nickel and Cobalt
said alloy further containing up to 2% of each of one or more
micro-alloying elements selected from the group consisting of:
zirconium, vanadium, cerium, titanium, tantalum, tungsten,
aluminum, niobium, calcium and rare earth elements with the balance
being essentially iron and other trace elements or inevitable
impurities and having a microstructure comprising chromium
carbides, borides and nitrides in an austenitic matrix, said matrix
being of face center cubic crystal structure, super saturated by
nitrogen in solid solution form and wherein the austenicity of said
alloy is defined by the following ratio ##EQU1##
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a High Chromium alloy and more
specifically to a corrosion and erosion resistant High Chromium,
nitrogen bearing castable alloy. The present invented alloy is
designed for use in the formation by casting of slurry pump parts,
such as casings, impellers, suction liners, pipes, nozzles,
agitators, valve blades, where the casting parts will be exposed in
highly corrosive fluids and abrasive slurries. A typical
application for such parts is in the wet processing of phosphoric
acid. Industrial phosphoric acid solutions are chemically complex,
containing sulfuric acid, hydrofluoric acid, hydrofluoric acid and
chlorides, fluorides and gypsum, all highly depassivating species,
very detrimental to the parts exposed. Another place where these
parts are used is in power plant scrubbers i.e., flue gas
desulfurization processes where the parts are exposed to sulfuric
components and gypsum.
One purpose of the present invention is to provide a material with
high resistance to chloride environments, at the same time the
material has extraordinary properties in acidic and basic
environments combined with good mechanical properties and high
structural stability. This combination can be very useful in
applications within for example the chemical industry, where you
have problems with corrosion caused by acids and at the same time
have a contamination of the acid with chlorides, which further
amplifies the corrosive effect. These properties of the alloy in
combination with a high strength lead to advantageous design
solutions from an economic point of view. There are certainly
existing materials with very good properties in acid environments,
but these are often steels with high contents of Ni, which makes
the costs of such materials excessively high. Another disadvantage
with austenitic is that the strength in the austenitic steel is
usually considerably low.
Applicant has found empirically that the solubility of Nitrogen in
a solid solution in the FerroChrome-Manganese Invented alloys is
0.013 to 0.0155% N maximum with 1% Chrome and minimum 6% Manganese,
and the same Molybdenum (2% Mo) as the best enhancement.
The Nitrogen has a much lower affinity to Chrome than Carbon has to
Chrome. The above-mentioned properties of Nitrogen in
High-Chrome-Manganese alloys cause the Carbon in those alloys to be
transformed into the Carbide phase, forming hard eutectic Chromium
carbides, with the surplus Carbon being dissolved together with
Nitrogen in the matrix.
Nitrogen introduced in a high concentration in solid solution
factors much stronger than Carbon on the sigma phase retardation,
allowing larger quantities of Chrome and Molybdenum to be dissolved
in the Ferro-Chrome-Manganese alloys to enhance passivation.
Nitrogen generally improves corrosion resistance, particularly in
Chloride containing media. In stainless steels its effectiveness
has been tested and expressed with the factor PREN (Pitting
Resistance Equivalent Number) -Cr %+3.3 Mo %+16N %. The higher the
level of the passivating elements (Cr, Mo, N), the higher the
resistance to the corrosion/erosion.
Additionally, Boron reacts with many elements in the periodic table
to form a wide variety of compounds. The strong covalent bonding of
most borides is responsible for their high melting points,
corrosion resistance and hardness values. The chemical resistance
of borides is superior to most either their nitride or carbide
counterparts. Because of the larger atomic size of B.apprxeq.0.91
.ANG., compared to C.apprxeq.0.77 .ANG. or N.apprxeq.0.71 .ANG.,
interstitial substitution of boron in the undistorted octahedral
site is rare, resulting primarily in boron-boron bonding, for
borides--M.sub.n B.sub.m. (NiB, CoB, MnB, FeB, CrB)
In addition, nickel, manganese and iron react strongly with boron
and form very hard compounds; much harder than their nitride or
carbides. For extremely abrasive and corrosive applications boron
should be added up to 5% B, the carbon content should be from 0.3%
C to 1.2% C and nitrogen 0.4 to 0.6% N.
Over all superior results are realized under this invention by the
novel microstructure, with the highly corrosive resistant matrix,
preferably austenitic, that is of face center cubic crystal
structure, super saturated by nitrogen in solid solution form. The
matrix is very hard, tough, non-brittle and embedded with borides,
carbides and nitrides, supporting the high corrosive resistant
matrix with highly wear resistance.
In practicing the instant invention, it is desired that the matrix
contain a high level of Chromium, Molybdenum and Nitrogen in a
solid solution, without Chromium, or Molybdenum combined by the
sigma phase precipitates. It is also desired that the invented
alloys have balanced its elements in accordance with the following
inequalities which is a measure of the invented alloy
austeniticity: ##EQU2##
According to the present invention, there is provided a corrosion
and erosion resistant Chromium-Nitrogen bearing castable alloy,
comprising the following composition in weight percent (wt %):
Chromium--28% to 48%
Nitrogen--0.01% to 0.7%
Manganese--0.5% to 30%
Carbon--0.3% to 2.5%
Boron--0.01% to 5%
Molybdenum--0.01% to 6%
Copper--0.01% to 8%
Nickel+Cobalt--0.01% to 25%
Silicon--0.01% to 5%
The alloy of the present invention may also contain up to 2% of an
additional element selected from a group consisting of: Zirconium,
Vanadium, Cerium, Titanium, Tantalum, Aluminum, Tungsten, Niobium,
Calcium, and rare earth elements with the balance being essentially
Iron and other trace elements or inevitable impurities.
A particular preferred alloy contains a range in wt. % of the main
elements (Chromium, Nitrogen, Manganese, Carbon, Boron, Molybdenum,
Copper, Nickel, Cobalt and Silicon) as follows:
Chromium 36%-42%
Nitrogen 0.45%-0.55%
Manganese 4%-15%
Carbon 0.5%-1.6%
Boron--0.01%-%4
Molybdenum--2%-5%
Copper--1%-6%
Nickel & Cobalt--4%-10%
Silicon--0.5%-1.5%
With the preferred composition it is desired that the austenitic
matrix contain 0.4 wt. % of solid solution of Nitrogen and 35 to
38% of Chromium plus Molybdenum plus Nitrogen.
Also, due to the targeted addition of the austenite-former nickel
and cobalt in the concentration range of 0.01 to 25 wt.-%, it is
possible to control the ratio of the ferrite and austenite phases
in the matrix in a defined manner. The normally extremely great
brittleness of chilled casting types with high carbon contents and
a carbide lattice in a ferritic matrix is avoided by the
predominant deposition of the chromium carbides in the only
austenitic phase. Since the austenitic phase, unlike the ferrite
phase, is not embrittled by segregation of intermetallic phases or
by segregation processes, the danger of fractures due to stresses
between the carbides and the matrix is not as great as it is in the
case of a purely ferritic or ferritic-austenitic matrix.
The molybdenum content within the limits 0.01% to 6 weight %,
preferably 2 to 4 weight %, and especially 2 to 3 weight %, is
important for corrosion resistance, especially in
chloride-containing, acidic media.
Also, by varying the alloy components carbon and chromium within
the limits 0.3% to 2.5% weight % for carbon and 28% to 48% wt % for
chromium, the corrosion resistance and wear resistance of the
material of the invention can be adjusted to correspond to a
prescribed profile of specifications.
The high chromium, nitrogen bearing alloy composition of the
present invention is also highly responsive to a cryogenic
hardening process, thereby becoming super-hard. When hardened by
the cryogenic treatment, the composition possesses higher abrasion
resistance, greater hardness, and a durable matrix without the
usual precipitation of secondary carbides.
The alloys of the invention are prepared by conventional methods of
melting, and no special conditions, such as controlled atmosphere,
special furnace linings, protective slags or special molding
materials are required.
In the treatment process of the present invention, the
high-chromium, nitrogen bearing castable alloy has many of the
alloying elements entirely distributed in the austenitic phase or
its transformation products, when subjected to sub-zero treatment
of at least -100.degree. F., preferably -100.degree. F. to
-300.degree. F., attain much greater hardening than that achieved
through conventional high temperature treatments.
Generally, the high-chromium, nitrogen bearing alloys of this
invention are made by preparing a molten metal mass of all the
required elements in the presence of air or additional nitrogen,
pouring castings therefrom, cooling of the castings, and subjecting
the castings to a cryogenic cooling treatment to produce the
desired hardness. The surface of the casting may be cleaned and
finished, either before or after cryogenic cooling. In more detail,
the preferred process involves the following steps: (1) mixing the
necessary components to be fed to the furnace; (2) melting the
mixture in the furnace to a pouring condition; (3) pouring the
molten metal composition into an appropriate mold; (4) letting the
mold and the casting therein cool slowly to room temperature under
ambient conditions; (5) cleaning and finishing the surface of the
casting, as by grinding or the like to smooth the surface; and, (6)
immersing the finished casting in a cryogenic cooling medium at a
temperature of -100.degree. F. to -300.degree. F. for a time
sufficient to reach the desired hardness.
To appreciate the present discovery, applicant conducted several
mechanical tests as further outlined below which included the
following measurements:
Tensile Strength--(Ksi)
Deflection--(mm), 30.5 mm diameter cast bar, 300 mm span.
Impact Energy_--(J), Izot test, unnotched 30.5 mm diameter bar,
struck 76 mm above support.
Hardness--(BHN): Brinell test, 3000 KG. Load on 10 mm tungsten
carbide boll. For the test, the preferred composition of alloys are
chosen from prior art alloys, the present invention and stainless
steel for reference.
The specific compositions tested are as follows:
Preferred composition alloys (in wt %) of U.S. Pat. No.
5,252,149
1 2 3 Cr 36.6 Cr 38.2 Cr 39.3 C 1.9 C 2.06 C 2.02 Mn 1.2 Mn 1.5 Mn
1.1 Si 1.5 Si 1.4 Si 1.5 Ni 2 Mo 1.2 Mo 1.8 Cu 1 Ni 1.2 Ni 1.6
Balance - Fe Cu 1.2 Cu 1.6 plus inevitable impurities Balance - Fe
Balance - Fe plus plus inevitable inevitable impurities
impurities
Preferred composition alloys (in wt %) of U.S. Pat. No.
5,320,801
4 5 6 Cr 29.8 Cr 32.7 Cr 34.8 Ni + Co 17.2 Ni + Co 26.5 Ni + Co
34.5 Si 3.4 Si 3.2 Si 3.5 Cu 1.9 Cu 3.1 Cu 3.8 C 1.65 C 1.28 C 1.26
Mn 1.1 Mn 1.5 Mn 1.6 Mo 0.9 Mo 1.8 Mo 2.2 Balance - Fe Balance - Fe
Balance - Fe plus plus plus inevitable inevitable inevitable
impurities impurities impurities
Present Invention Alloys in Wt %
7 8 8B 9 Cr 35.8 Cr 37.3 Cr 37.9 Cr 38.3 N 0.42 N 0.48 N 0.4 N 0.52
Mn 6.1 Mn 9.8 Mn 5.2 Mn 11.1 C 1.26 C 1.33 C 1.33 C 1.41 B 0.2 B
0.15 B 3.8 B 0.1 Mo 3 Mo 2.6 Mo 2.6 Mo 2.2 Si 0.9 Si 0.8 Si 1 Si
0.7 Cu 1.5 Cu 1.7 Cu 1 Cu 1.9 Co 2.1 Co 0.6 Co 0.5 Co 4 Ni 3.25 Ni
3.6 Ni 8.2 Ni 0.2 Balance - Fe Balance - Fe Balance - Fe Balance -
Fe plus plus plus plus inevitable inevitable inevitable inevitable
impurities impurities impurities impurities
Alloy Compositions in Wt % of German Pat. 8612044, 4417261
10 11 12 Cr 38.8 Cr 43 Cr 44 Ni 5 Ni 8 Ni 10 Mo 2 Mo 3 Mo 3.5 Cu 2
Cu 2.5 Cu 2.1 N 0.19 N 0.09 N 0.15 Si 1 Si 1.5 Si 1.5 Mn 1 Mn 1.2
Mn 1.1 C 1.6 C 1.7 C 1.6 V 1.2 Balance - Fe Balance - Fe Balance -
Fe plus inevitable plus inevitable plus inevitable impurities
impurities impurities
Stainless Steel Alloy Compositions in Wt % for mechanical
Testing
20Cb3 Cd-4MCu + N 317L Cr 20 Cr 26.5 Cr 18 Ni 37.5 Ni 5.5 Ni 11 Mo
3 Mo 2.5 Mo 3.1 Cu 3 Cu 2.9 C Min. Nb 0.4 N 0.23 C Min C Min
Balance - Fe Balance - Fe Balance - Fe plus inevitable plus
inevitable plus inevitable impurities impurities impurities
TABLE 1 Tensile Sample No. Strength Elongation Deflection Impact
Hardness U.S. Pat. No. (Ksi) % (mm) (J) (BHN) Comments 5,252,149 1
61 0 2/3 12 19 450 as cast 2 64 0 1.3/1.9 11 18 460 3 58 0 0.9-1.9
10 16 490 Heat treatment at 1450.degree. F. for 3 hrs 5,320,801 4
53 0 8-11 22-26 360 Sample: 5 54 0.3-0.6 9-13 26-34 330 Hardened at
1400.degree. F. for 4 hrs 6 48 0.3 0.5 8-13 22 31 320 Hardened at
1400.degree. F. for 4 hrs Present invention 7 95 0.5-1.1 14-18
48-59 512 Cryogenic C hardened at -300.degree. F. 8 111 0.4-1.0
10-16 41-49 450 Heat Treated .sup. 8B 109 0 8-12 30-36 530 As cast
9 95 0.3-0.6 9-12 36-47 490 as cast German Patents 4,417261,
8,612,049 10 68 0 1.5-2.2 11-16 500 Heat treatment at 1800.degree.
F. for 2 hrs 11 65 0 1 2.0 10-15 450 12 64 0 0.6 1.6 8-14 490
The alloys 1, 2, 3, 10, 11 and 12 of the prior art have eutectic
microstructure where the matrixes are essentially ferritic
(Fe-.alpha.).
The German Patent 4,417,261, or 8,612,044, alloys identified as 10,
11 and 12, claim a maximum of up to 40% or Fe-.alpha. in the
matrix. The phase of Fe-.alpha. in the High Chrome alloys
inherently posses very low toughness because of the very low
solubility of Carbon and Nitrogen in the Fe-.alpha.. Even a small,
limited addition of Nitrogen has a detrimental effect on the
toughness, deflection and heat sensitivity, making the alloy more
brittle.
Alloys 4, 5 and 6 of U.S. Pat. No. 5,320,801 are Chrome high Nickel
alloys with an austenitic microstructure. Those high Nickel alloys
inherently possess the lowest tensile strength, the lowest
hardness, as cast above 200 HB, and after hardening from the range
of 300 HB, they lose their toughness and corrosion resistance.
As can be appreciated from Table 1 above, the alloys of the present
invention 7, 8 and 9 possess the following properties superior to
prior art alloys:
2 to 3 times greater toughness
1.6 to 2.3 times higher tensile strength
Very high as cast hardness after cryogenic hardening
Measurable elongation or malleability
Excellent deflection
1.5 to 2.5 higher max. hydraulic pressure vessel test.
Low heat sensitivity
Good machinability, especially threadability, which on prior art
alloys was very poor
Best castability with melting and pouring temp. -150.degree. F.
lower
The alloys of the prior art as well as the alloys of the present
invention are subjected to corrosion test to show the superiority
of the alloys of the instant invention:
The Corrosion Tests are conducted in synthetic P.sub.2 O.sub.5 acid
at 80.degree. C., with a chloride content of from 1000 to 3000 ppm.
Agitated, 96 hr test. (mmy). The results of the corrosion tests are
summarized in Table 2.
TABLE 2 Chloride Corrosion PREN = Sample No. Hardness Content Rate
% Cr + 3.3 .times. Patent No. (BHN) (PPM) (mmy) % Mo + 16 .times. %
N U.S. Pat. No. 260 1000 17 PREN.sub.5 = 38 5,320,801 2000 28 5
3000 56 As cast 5 330 1000 23 Hardened 2000 36 At 1400.degree. F./
3000 65 4 hr U.S. Pat. No. 460 1000 15 PREN.sub.2 = 42 5,252,149
2000 23 2 3000 49 as cast Present 450 1000 8 PREN.sub.8 = 53
Invention 2000 11 8 3000 16 As Cast Stainless Steel 180 1000 13
PREN = 30 20Cb-3 2000 14 (20Cb-3) 3000 32 Stainless Steel 280 1000
11 PREN = 38 CD-4MCuN 2000 15 3000 19 CD-4MCuN 330 1000 17 CD-4MCuN
Hardened 2000 28 3000 45 Stainless Steel 185 1000 0.68 PREN = 38
317L 2000 1.1 (317L) 3000
The following conclusions can be drawn from Table 2:
The High Chrome alloy No. 5 of U.S. Pat. No. 5,320,801 containing
-26% Nickel, has a lower corrosion resistance than alloy No. 2 of
prior art U.S. Pat. No. 5,252,149, where Nickel content is only
1%.
The same conclusion applies to the stainless steel alloy 20Cb3, in
which the Nickel content is 37%. The alloy CD4MCuN contains only 5%
Ni. The main function of Nickel in corrosion resistant alloys is as
a structural component.
The No. 8 High Chrome-Nitrogen bearing alloy of the present
invention, contains only 3.6% Nickel, but 0.48% Nitrogen which is a
very powerful corrosion inhibitor. Nitrogen interacts with the
Chlorides and somehow buffers their detrimental effect on the
alloy. The present invented alloy No. 8 with the higher PREN=53,
has 2 to 3 times better corrosion resistance than the patented
alloys No. 5 and No. 2. Alloy No. 8 of the present invention
containing high levels of Chrome, Molybdenum with a high
concentration of Nitrogen, possesses the best corrosion resistance
in acidic environments containing high levels of Chlorides.
Prior art alloys and the alloys of the present invention are also
subjected to corrosion erosion tests as shown below.
Corrosion Erosion test
The corrosion erosion tests are done using 30% by weight 80 microns
alumina suspended in 28% P.sub.2 O.sub.5 synthetic acid, 1.5%
H.sub.2 SO.sub.4, 0.05% hydrofluoric acid plus 1000 ppm Cl,
temperature 800.degree. C., Rotation 650 RPM, Duration 12 hr. Mass
loss (mg). The results of erosion corrosion testing are tabulated
in Table 3 below.
TABLE 3 Weight PREN = Hardness Loss % Cr + 3.3 .times. Sample No.
BMN (mg) % Mo + 16 .times. % N U.S. Pat. 5,320,801 260 306.6 PREN
(5) = 38 5 as cast 5 330 282.6 age hardened at 1400.degree. F./4
hr. Present invention 8 - B 530 96.3 PREN (8B) = 53 8 450 123.3
PREN (8) = 53 as cast 8 anneal/S solution at 450 125.1 2000.degree.
F./4 hr. Stainless Steel 280 426 PREN = 38 CD4MCuN Solution
(CD-4MCuN) Anneal CD-4MCuN 330 328.2 Age hardened 20Cb-3 180 660.3
PREN = 30 solution anneal (20Cb-3)
The slurry corrosion-erosion tests indicate that the most of the
mass is lost from alloy 20Cb-3, which has the lowest hardness.
Prior art alloy No. 5 has a low hardness, comparable to the
hardness of the reference stainless steel CD-4MCuN.
The loss of mass on the sample No. 5 alloy of U.S. Pat. No.
5,320,801 is 50% less than on the sample of the stainless steel
alloy Cd4MCuN. On the present invented alloy sample No8, the loss
of mass is 245% less than on the reference alloy Cd4MCuN. The
present invented alloy No.8 with the highest PREN factor=53,
possesses the highest corrosion-erosion resistance .about.3.5 times
better than the reference alloy CD4MCuN and 2.3 times better than
alloy No.5 of U.S. Pat. No. 5,320,801.
The present invented alloy with boron No.8B with the highest
hardness and PREN=53 possess the highest corrosion-erosion
resistance .about.4.4 times better than the referenced alloy
CD-4MCuN and 2.9 times better than alloy No.5 of the U.S. Pat. No.
5,320,801.
Any conventional or under nitrogen partial pressure casting
technology may be used to produce the alloys of the present
invention.
It is preferred that the alloys are formed by any conventional
casting technology and then heat treated at a temperature in the
range of 1800.degree. to 2000.degree. F., followed by air
cooling.
The most preferred hardening method for the alloy of the present
invention is by cryogenic treatment: cooling to at least from
-100.degree. F. to -300.degree. degrees F., and maintaining at
those temperatures for a time of one hour per one inch of casting
wall thickness.
The cryogenic tempering process is performed with equipment and
machinery which is conventional in the thermal cycling treatment
field. First, the articles-under-treatment are placed in a
treatment chamber which is connected to a supply of cryogenic
fluid, such as liquid nitrogen or a similar low temperature fluid.
Exposure of the chamber to the influence of the cryogenic fluid
lowers the temperature until the desired level is reached. In the
case of liquid nitrogen, this is about -300.degree. F. (ie.,
300.degree. F. below zero).
Various changes and modifications may be made within the purview of
this invention, as will be readily apparent to those skilled in the
art. Such changes and modifications are within the scope and
teachings of this invention as defined by the claims appended
hereto. The invention is not to be limited by the examples given
herein for purposes of illustration, but only by the scope of the
appended claims and their equivalents.
* * * * *